A MOSFET current mirror is an essential component of integrated circuit amplifiers that is used to implement current sources for biasing and may also operate as an active load. The MOSFET current mirror typically includes at least two devices configured such that the ratio of currents through each device remains largely constant. The current ratio is controlled by the physical geometry of the transistors, which enables the current flowing through an output device to be approximated by reference to the current flowing through a reference device. In this regard, current in the output device is proportional to the current in the reference device, thereby “mirroring” the reference current.
A current reference circuit producing a stable output current in the presence of fluctuations in the voltage applied at its output is useful in analog circuits where variables may be expressed as a simple current, a ratio of currents, or a biased reference current. To stabilize the output current, many current reference circuits incorporate some form of feedback. The reference and output transistors of a typical current mirror have non-linear current versus voltage characteristics that are well matched, thereby producing a current ratio that is ideally constant over a wide output voltage range.
However, MOSFET transistors are rendered imperfect current sources because a voltage applied to the drain—typically the output when the transistor is used as a current source—causes a modulation of the size of the drain-channel depletion region. As the drain voltage increases, the size of the depletion region grows and the effective channel length is decreased. As a result, the drain current increases as well, hence degrading operation of the device as a constant current source. This tendency can be determined from the saturated drain current equation:Id=½(μnCox)·(Weff/Leff)·(Vgs−Vt)2 
where Id clearly increases as Leff decreases. In general, Leff is regarded as fixed and another term is added to the equation to account for channel length modulation:Id=½(μnCox)·(Weff/Leff))·(Vgs−Vt)2·(1+λVds)
that models the dependency of Id on Vd as a linear approximation.
There are two prior art approaches in dealing with the undesirable change in drain current associated with modulation of the drain depletion region. One is to simply make the design channel length larger, which lessens the effect of the depletion region modulation. The change in dimension of the depletion region is a fixed function of the drain voltage and drain doping but not of the channel length. This has the effect of reducing the value of λ in equation 2 above and “flattening” the device curves in the saturation region. However, this technique suffers from either an increase in area with the square of the increase in Leff (since Weff needs to increase by the same proportion) or an increase of the voltage bias margin required for the current source to operate properly in the saturation region.
Another technique is to add circuitry to the basic MOSFET current mirror that will increase the output resistance. There are literally dozens of circuit topologies designed to provide higher output impedance, the simplest and most straightforward of these being to place a common-gate cascode device immediately in series with the drain of the current mirror. This has the effect of isolating the drain of the current mirror from variations in the voltage at the output of the mirror circuit; the drain observes a voltage set only by the cascode gate bias and the cascode gate-to-source voltage, which is a weak function of the current through the device. Unfortunately, this technique has the disadvantage of requiring additional circuit area and an additional voltage bias margin across the aggregate mirror structure (the mirror and cascode devices) in order for the cascode device to function properly.
Accordingly, a need exists for a current mirror with improved output impedance characteristics that does not present a significant impact to the area and voltage bias margin of the current mirror device.